Recently on DigitalPoint, I posted in response to someone who had some old domains forwarding email to his Gmail account - high traffic domains that received tens of thousands of spam messages per day.
I presented him a solution, which might work for you, too, if you're in a similiar situation.
Up at the top of your Gmail, click "Create a Filter". It's to the right of the search box.
In the "From:" box, type the domain name of the affected domain. Click "Next Step".
Select "Delete It". Click "Create Filter" and you're done.